This invention relates to an erasable optical data storage medium having an embedded servo track, and particularly to such a medium having a subsurface recording interface including a dyed polymer material.
Many of the known optical data storage media are of the "air incident" type as exemplified by Cornet U.S. Pat. No. 4,404,656, Howe U.S. Pat. No. 4,336,545, and Bell U.S. Pat. No. 4,285,056, wherein the optically detectable mark occurs at the surface of the medium. Since scratches, dirt or the like on the recording surface of the medium would interfere with the recording and/or reading and/or erasing of data on the recording surface, one common method of protecting the recording surface of the medium is to space a hard transparent cover above the recording surface of the medium. This technique is sometimes referred to as "dust defocusing" since it serves to separate dust and dirt away from the recording surface and out of the focal plane of the operative laser beam. Not only does the resulting air space and cover increase the bulk of the resulting disk assembly, but spacing the cover precisely and evenly above the recording surface of the medium is a manufacturing problem which requires considerable care and expense.
Another method of protecting the integrity of the recording surface and providing a dust defocusing structure is to cover the recording surface with a protective layer which will not interfere with the formation of an optically detectable mark. Examples of media which employ a subsurface recording interface covered by a protective layer are shown in Cornet, U.S. Pat. Nos. 4,398,203, 4,577,291, Willis U.S. Pat. No. 4,264,986 and Japanese patent application Ser. No. 55-136253. As shown in the Japanese patent application the soft protective layer may also be covered by a hard protective layer.
In either case format data and/or tracking data can be incorporated into the storage medium or the medium assembly. The practice of recording such format and tracking data directly into the medium, although widely used, is very time-consuming. While compact discs (CD) employ hard, transparent substrates which are stamped or molded with ROM (read only memory) data, this technique is not presently appropriate for air incident, covered, writable disks because the requisite air space between the recording surface and the cover does not permit the prerecorded data on the cover to be read simultaneously in the same focal plane with the data recorded in the medium.
Applicant is aware of a medium disclosed by Ahn et al. U.S. Pat. No. 4,578,788 which addresses some of the aforementioned problems by employing a substrate including a servo track containing preformatted servo information embedded in the medium in close proximity to the data recording layer. The laser beam(s) used to "write" and "read" pass through the substrate to the data recording layer, a portion of the read beam being reflected from the embedded servo track and detected to provide servo information. Such media with embedded servo tracks are sometimes called "substrate incident" media. It should be noted that the medium disclosed by Ahn et al. is not an erasable medium--the data mark is formed by a vesicular or ablative technique which cannot be reversed to erase the data mark. It should also be noted that Ahn et al. relies upon a metal or metallic layer to write, the metallic layer absorbing light energy from the laser beam to heat an adjacent polymer layer, causing it to expand and deform the metallic layer. The metallic layer also reflects the light of the laser beam to read the data mark.
Three patent applications commonly owned by applicant's assignee disclose erasable optical data storage media adapted to write and erase air-incident, optically-detectable deformations in a non-ablative, non-vesicular manner. Like the present invention, these applications are directed toward a medium which uses dyed polymer materials to create zones or layers of differing optical and thermal-mechanical properties which may be operatively acted upon by a laser beam of a predetermined wavelength to write data in the medium and to read data in the medium, and acted upon by another laser beam of predetermined wavelength to erase data from the medium. Pursuant to MPEP 608.01(P), commonly owned U.S. patent application Ser. Nos. 835,960, 914,461 and 916,609 are incorporated herein by reference with particular attention drawn to the identification of the prior art contained therein and to the media and methods disclosed therein.